You need to first find a mineral vein. They are Maple Story Mesos on hills, rocky surfaces, cliffs, and in caves. You can use the skill "Find Minerals" which will show any nearby mineral veins on your mini-map. They will be small yellow dots and if you hover your mouse over them you will see what kind of mine they are. As a small note, tracking abilities such as Find Herbs, Track Humans, and Sense Undead will be overridden when you use Find Minerals. You can only have one type of tracking/finding ability going at once.